Analog optical switch using an integrated Mach-Zehnder interferometer having a moveable phase shifter

ABSTRACT

Optical devices employing a Mach-Zehnder interferometer (“MZI”) device having a phase shifter provided in one arm that enables the construction of significantly smaller optical devices than typical photonic devices, and significantly reduces the amount of on-chip real estate occupied by such devices, while not affecting the ability of such devices to introduce a predetermined phase shift in an optical signal. The present invention takes advantage of the extremely small mechanical actuators which can be fabricated using small-scale fabrication techniques, and so significantly reduces the room needed on a chip for optical switches. These more compact switches require less chip space and so provide for denser integration of a plurality of optical devices in an optical component.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to Provisional Patent Application No.60/167,447, filed on Nov. 23, 1999.

FIELD OF THE INVENTION

The present invention is directed to small-scale Mach-Zehnderinterferometer (“MZI”) devices and structures. The present invention isalso directed to analog optical switches.

BACKGROUND OF THE INVENTION

An optical network, in its simplest representation, consists of anoptical source, a destination, and a matrix of devices (e.g.,fiber-optical cables, waveguides, cross-connects, amplifiers, etc.) forcausing an optical signal generated by the source to reach a desireddestination. Physical and geographic boundaries present no impediment totelecommunication, data communication and computing, all of which mayutilize all or part of an optical network. Consequently, the number orsources and destinations, and the combinations of sources anddestinations and the communication paths therebetween, may be nearlyinfinite. Optical switches are used in the optical network forfacilitating the routing of an optical signal to its desireddestination.

By way of example, FIG. 1 depicts a block diagram of a part of anoptical component 1 comprising a plurality of optically interconnectedoptical devices 3 (e.g., switches, filters, etc.), shown in FIG. 1 asswitches. As used herein, the terms “optical component” and “component”refer to any and all of a plurality of interconnected devices which mayoperate using any combination of optical, opto-electrical, and/orelectrical technologies and which may be constructed as an integratedcircuit. Devices 3 can be optically interconnected by waveguides 5.Various other optical, opto-electrical, and/or electrical devices mayalso be included in the optical component, as a matter of design choice.As used herein, the terms “optical”, “opto-electrical”, and “electrical”devices may include, by way of non-limiting example, lasers, waveguides,couplers, switches, filters, resonators, interferometers, amplifiers,modulators, multiplexers, cross-connects, routers, phase shifters,splitters, fiber-optic cables, and various other optical,opto-electrical, and electrical devices. The optical component 1 anddevices 3 depicted in FIG. 1 are merely illustrative.

Although a single wavelength of light can be transmitted through thenetwork, in order to increase the network's data-carrying capacity it ispreferable to transmit multiple wavelengths of light at the same time.This is currently accomplished using techniques known aswave-division-multiplexing (“WDM”), dense WDM (“DWDM”), and ultra-densewave-division-multiplexing (“UDWDM”).

The ability to separate one optical signal from a plurality of opticalsignals (or one wavelength from a plurality of wavelengths in an opticalsignal) propagating within an optical network becomes more important asthe number of signals transmitted through a single optical fiber (orwaveguide) increases. As optical transmission evolves from WDM to DWDMto UDWDM, and beyond, more and more data contained in a multi-wavelengthoptical signal is transmitted over the optical network. Optical filtersare one component that may be used to extract a desired signal (i.e., adesired wavelength) at a particular point or location in the network androute that signal to its desired destination, while also permittingother signals to continue along the network.

Optical networks transmit data as pulses of light through waveguides ina manner similar to electrical networks, which send pulses ofelectricity through wiring. Transmitting an optical signal betweenwaveguides, which may occur in various devices employed in an opticalnetwork, may require the optical signal to leave one waveguide andpropagate through one or more materials (mediums) before enteringanother waveguide. It is likely that at least one of the devices willhave an index of refraction different than the index of refraction ofthe waveguides (which typically have approximately the same refractiveindex). It is known that the transmission characteristics of an opticalsignal may change if that signal passes through materials (mediums)having different indices of refraction. For example, a phase shift maybe introduced into an optical signal passing from a material having afirst index of refraction to a material having a second index ofrefraction due to the difference in velocity of the signal as itpropagates through the respective materials and due, at least in part,to the materials' respective refractive indices. As used herein, theterm “medium” is intended to be broadly construed and to include avacuum.

If two materials (or mediums) have approximately the same index ofrefraction, there is no significant change in the transmissioncharacteristics of an optical signal as it passes from one material tothe other. Accordingly, one solution to the mismatch of refractiveindices in an optical switch involves providing an index matching orcollimation fluid to offset any difference in refractive indices.Consequently, the optical signal does not experience any significantchange in the index of refraction as it passes from one waveguide toanother.

An example of this approach may be found in international patentapplication number WO 00/25160. That application describes a switch thatuses a collimation matching fluid in the chamber between the light paths(i.e., between waveguides) to maintain the switch's optical performance.The use of an index matching fluid introduces a new set of designconsiderations, including the possibility of leakage and a possibledecrease in switch response time due to the slower movement of theswitching element in a fluid.

In addition, the optical signal will experience insertion loss as itpasses between waveguides. A still further concern is optical returnloss caused by the discontinuity at the waveguide input/output facetsand the trench. In general, as an optical signal passes through thetrench, propagating along a propagation direction, it will encounter aninput facet of a waveguide which, due to physical characteristics ofthat facet (e.g., reflectivity, verticality, waveguide material, etc.)may cause a reflection of part (in terms of optical power) of theoptical signal to be directed back across the trench (i.e., in adirection opposite of the propagation direction). This is clearlyundesirable because the reflected signal will interfere with the opticalsignal propagating along the propagation direction.

Reflection of the optical signal back across the trench also can createproblems if the facets not only are coaxial, but also are parallel toone another. That arrangement forms a Fabry-Perot resonator cavity,which, under the appropriate circumstances, allows for resonance of thereflected signal, in known fashion.

Size is also an ever-present concern in the design, fabrication, andconstruction of optical components (i.e., devices, circuits, andsystems) for use in optical networks. It is strongly desirable toprovide smaller optical components so that optical devices, circuits,and systems may be fabricated more densely, consume less power, andoperate more efficiently.

Currently, optical switches can be constructed using a directionalcoupler or a Mach-Zehnder interferometer (“MZI”), as is generally knownin the art. Mach-Zehnder interferometers are known devices which take aninput optical signal, split the signal in half (generally, in terms ofoptical power), direct the split signals along different optical paths,apply a phase shift to one of those split signals, recombine the signalsand then feed those combined signals as a single signal to an output.The amount by which the phase of one of the signals is changed will, inknown fashion, affect the nature of the output signal.

Conventional Mach-Zehnder interferometers shift the phase of lighttraveling along one of the interferometers in one of several ways. Ifthe electro-optic effect is used, one of the interferometer arms is madefrom a medium having an index of refraction which changes in thepresence of an applied electrical field. Similarly, if theelectro-thermal effect is used, the interferometer has an arm made froma medium having an index of refraction that changes as the temperatureof the material changes. In each of these devices, changing the index ofrefraction of one of the interferometer arms is comparable to changingthat arm's optical length, and results in a relative phase shift betweenthe two split signals. In another known type of MZI, one of the twointerferometer arms is actually longer (and thus, optically longer) thanthe other, and this also results in a relative phase shift betweensignals propagating in each arm.

In the electro-optic and electro-thermal type devices, the conditionsfor effecting optical switching in a device using a MZI, which operatesby introducing a phase shift of up to π (i.e., 180°) into at least apart of the optical signal, are defined by the equation: $\begin{matrix}{{\Delta \quad \varphi} = {\pi = {\frac{2\quad \pi}{\lambda}\Delta \quad {nL}}}} & (1)\end{matrix}$

where Δø is the maximum possible phase shift of π, λ is the wavelengthof the optical signal propagating in the device, L is the actual lengthof the device, and Δn is the change in refractive index effected by theapplication of a carrier signal, electrical field, or change intemperature to the device. Since the change in refractive indextypically achievable for current optical devices is on the order ofapproximately 10⁻³, the actual length of the device needed to introducea maximum phase shift of π must be at least 1 mm, and preferably longer.However, to achieve large-scale density integration, the actual length Lmust be reduced without sacrificing the ability to effect a π phaseshift in an optical signal. Those two requirements are mutuallyexclusive.

If the phase is to be applied using a MZI device having different lengtharms, the light traveling through the longer arm has its phase shiftedrelative to the light passing through the other arm. Because of thedifference in arm lengths, this technique cannot be used to make compactoptical switches.

There exists a genuine need in the art for compact optical switches thatcan effect a 0-π phase shift and which overcome the above-describedshortcomings of the prior art. Preferably, such switches would combinesmall size and high actuation speed with low power consumption.

SUMMARY OF THE INVENTION

The present invention is directed to an analog optical switch having aMZI with a moveable phase shifter in one interferometer arm suitable foruse in an optical network.

More particularly, this invention is directed to improved analog M×Mswitches which employ Mach-Zehnder interferometers to control opticalsignals. As already explained, MZI devices operate by dividing an inputoptical signal into two signals, applying a phase shift to just one ofthose signals, and then recombining the two signals. The output willdepend upon the magnitude of the phase shift applied. As notedpreviously, known switches of this type are larger than desired becausethe MZI devices used therein operate using techniques which thwartminiaturization.

Switches according to the present invention differ from known opticalswitches because of the unique MZI provided in accordance with thepresent invention. A MZI constructed in accordance with embodiments ofthe present invention includes a phase shifter in one interferometerarm. The phase shifter is selectively moveable into and out of anoptical path defined by and through the interferometer arm so as tointroduce a predetermined phase shift into an optical signal propagatingin and through that interferometer arm. This arrangement dramaticallyreduces the size of the MZI as compared with conventional opticalswitches, which may employ the electro-optic, electro-thermal orasymmetric arms to introduce a phase change in an optical signal. A MZIusing a phase shifter in accordance with this invention is far morecompact than a MZI which uses those known techniques.

The present invention is particularly applicable to optical switchesthat are formed on integral planar optical substrates. Generallyspeaking, an integrated planar optical substrate refers to a relativelyflat member having a supporting substrate and a number of layers ofdifferent materials formed thereon. The substrate and the differentmaterials have particular optical qualities so that optically usefulstructures such as waveguides can be formed on the supporting substrateby suitable shaping or other processing. Such optical switches may bemore compact and more rapidly actuated than comparable known devices.

As explained in greater detail below, this invention involves phaseshifters constructed using small-scale fabrication techniques. Thisinvention also encompasses phase shifters made using other fabricationstechniques which result in comparable devices.

The present invention takes advantage of the extremely small mechanicalactuators which can be assembled using small-scale fabricationtechniques, and so significantly reduces the room needed on a chip foroptical switches. These more compact switches require less chip spaceand so provide for denser integration of a plurality of optical devicesin an optical component. This invention also takes advantage of thestrong photon confinement properties of small-scale waveguides, such asare disclosed in U.S. Pat. Nos. 5,878,070 and 5,790,583. Together thesedevelopments facilitate construction of optical devices that provide thebenefits and advantages of the present invention.

One embodiment of the present invention involves a Mach-Zehnderinterferometer having a single input, a single output, and first andsecond arms extending along an optical path direction of theinterferometer. One arm has a phase shifter disposed therein. When thephase shifter is actuated an optical signal propagating through the armhaving the phase shifter will experience a phase shift relative to anoptical signal propagating through the other arm.

In accordance with the present invention, a MZI may be constructed witha selectively moveable phase shifter in one interferometer arm. Thatphase shifter may be moved into and out of an optical path defined byand through that interferometer arm so as to introduce a phase shiftinto an optical signal propagating in and through that arm. In so doing,the phase shifter changes the optical length of that arm, when comparedwith the optical length of the other interferometer arm. The phaseshifter may be generally wedge-shaped, rectangular, square, stepped (onone or both sides), or other shapes, provided that such shapes may beutilized to introduce a phase shift into the optical signal.

While it is generally known to provide a MZI as an element of an opticalswitch, a MZI constructed in accordance with the embodiments of thepresent invention provides significant advantages over prior art MZIdevices and optical switches. For example, the micron-scale of the MZIenables construction of smaller optical switches that consume lesson-chip real estate. The power requirements of the MZI to effect adesired phase change in an optical signal are also significantly reducedwhen compared with prior art MZI devices.

The invention accordingly comprises the features of construction,combination of elements, and arrangement of parts which will beexemplified in the disclosure herein, and the scope of the inventionwill be indicated in the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawing figures, which are not to scale, and which are merelyillustrative, and wherein like reference characters denote similarelements throughout the several views:

FIG. 1 is a schematic block diagram of a 1×16 switch that is part of ahigh-density optical component;

FIG. 2 is a schematic diagram of a 1×1 optical switch having aMach-Zehnder interferometer constructed in accordance with the presentinvention;

FIG. 3 is a schematic diagram of a 2×2 optical switch having aMach-Zehnder interferometer constructed in accordance with the presentinvention;

FIG. 4 is a cross-sectional view of a photonic-wire waveguide;

FIG. 5A is a perspective view of a phase shifter constructed inaccordance with an embodiment of the present invention; FIG. 5B is across-sectional view taken along line 5B—5B of FIG. 5A;

FIG. 6 is a cross-sectional view showing an alternate configuration tothat shown in FIG. 5B;

FIGS. 7A and 7B are top schematic views showing a phase shift elementpositioned out of and in an optical path defined by and through twowaveguides;

FIG. 8 is a top view of a tapered phase shift element constructed inaccordance with an embodiment of the present invention;

FIG. 9A is a top cross-sectional view of an alternate embodiment of aphase shift element constructed in accordance with the presentinvention;

FIG. 9B is a top cross-sectional view of still another embodiment of aphase shift element constructed in accordance with the presentinvention;

FIG. 9C is a front elevational view of the phase shift element viewed ofFIG. 9A as viewed along line 9-9;

FIGS. 10A and 10B are schematic views showing ways to reduce diffractionof light passing between waveguides;

FIG. 11 is a schematic view showing offset waveguides arranged about aphase shift element;

FIG. 12 is a schematic view showing waveguides with angled facets;

FIGS. 13A and 13B depict the assembly of an optical switch in accordancewith an embodiment of the present invention

FIGS. 14A and 14B show the relationship between light beam diffractionand trench width for light passing across a trench between waveguides;and

FIGS. 15A and 15B are partial side cross-sectional views showingportions of the structure of optical switches in accordance with thepresent invention manufactured using flip-chip and monolithicfabrication techniques, respectively, together with external componentsand connecting hardware.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

The present invention is directed to optical devices employing aMach-Zehnder interferometer (“MZI”) device having a phase shifterprovided in one interferometer arm. The present invention enables theconstruction of significantly smaller optical devices than typicalphotonic devices, and significantly reduces the amount of on-chip realestate occupied by such devices, while not affecting the ability of suchdevices to introduce a predetermined phase shift in an optical signal.

The present invention takes advantage of the extremely small mechanicalactuators which can be fabricated using small-scale fabricationtechniques, and so significantly reduces the room needed on a chip foroptical switches. These more compact switches require less chip spaceand so provide for denser integration of a plurality of optical devicesin an optical component.

As used herein, an “analog switch” is a switch having more than twooutput states. The term “light” as used herein should be construed inthe broadest possible sense. For example, the term “light” is intendedto include visible electromagnetic radiation, as well as infrared andultraviolet radiation. The term “waveguide”, as used herein, refersgenerally to a photonic-well or photonic-wire structure that providesstrong photon confinement. The term waveguide is not intended as alimitation on the construction, shape, materials, functionality, or anyother aspect of the optical device of the present invention, but merelyas a general reference.

Referring now to the drawings in detail, and with initial reference toFIG. 2, a Mach-Zehnder interferometer (MZI) 13 is used to construct a1×1 optical switch 7. The switch 7 may receive at an input 27 an opticalsignal input via an input waveguide 49 and from an optical source 23,which by way of example and not limitation may include a laser,fiber-optic cable, or other upstream (along the optical path direction)light generating or light propagating device or system. The opticalsignal may be a single- or multi-wavelength signal. An output 31 viaoutput waveguide 49′ is controllable by the MZI 13, as described in moredetail below.

The MZI 13 has first and second arms 15, 17 optically connecting theinput 27 through input waveguide 49 to output waveguide 49′ and theoutput 31. Light travels along an optical path defined by and throughthe input and output waveguides 49, 49′ and the MZI arms 15, 17 in adirection generally indicated by arrow A in FIG. 2. In contrast to knownMZI devices, present invention provides a phase shifter 51 in one MZIarm to introduce a phase shift into an optical signal propagating in andthrough that arm and thus control the output 31 of the MZI 13 and anoptical switch 7 that includes the inventive MZI 13.

With continued reference to FIG. 2, the optical signal traveling alonginput waveguide 49 splits approximately and preferably equally (in termsof signal amplitude or power) to each of the first and second arms 15,17 of the MZI 13. After passing through each of the first and secondarms 15, 17, the divided optical signal is recombined and thentransmitted along output waveguide 49′ to output 31.

Notwithstanding the unique construction of the phase shifter formed inthe second arm 17 provided by the present invention and described below,the MZI 13 of the present invention functions in a fashion similar toother Mach-Zehnder devices. For example, a phase shift ranging fromapproximately 0° to approximately 180° (π) may be introduced into anoptical signal propagating in the interferometer's second arm 17 whenthe phase shifter is actuated to alter the phase of light travelingthrough that second arm 17, and this will determine how light is outputfrom the switch 7. The mechanism through which such a 1×1 MZI switchroutes light passing from the input to the output paths is known, and soneed not be described.

In accordance with the present invention, a phase shifter 51 is providedin one interferometer arm 17 (which arm is a routine matter of designchoice). With continued reference to FIG. 2, and with additionalreference to FIGS. 5A and 5B, one embodiment of the phase shifter 51 ofthe present invention will now be described in detail. Theinterferometer arm 17 in which the phase shifter 51 is provided isnon-continuous, although defining a continuous optical path. Thus, thatinterferometer arm 17 comprises two waveguides 117, 217, separated by aregion or trench 59 within which is provided the phase shifter 51. Anoptical signal propagating in and through the interferometer arm 17 willpass through phase shifter 51 or not, depending upon whether phaseshifter 51 is positioned in or out of the optical path defined by andthrough that waveguide 117, trench 59 and waveguide 217. When the phaseshifter 51 is positioned in the optical path, the phase of an opticalsignal propagating in and through interferometer arm 17 may be changed,depending upon the position and construction of the phase shifter 51, asdescribed in more detail below.

In one embodiment of the present invention, depicted in FIGS. 1, 5A, 5Band 6, the phase shifter 51 comprises a tapered or wedge-shaped phaseshift element 53 connected to an actuator 55 by a link 57. That actuator55 and link 57 may cause selective movement of the phase shift element53 into and out of the optical path, thus changing the phase of anoptical signal propagating in and through interferometer arm 17 (andwaveguides 117, 217). For example, when the phase shift element 53 ispositioned as depicted in FIG. 7A (i.e., out of the optical path), anoptical signal propagating in and through the interferometer arm 17 willpass from waveguide 117 to waveguide 217 across trench 59 withoutexperiencing a phase shift. On the other hand, when the phase shiftelement 53 is positioned as depicted in FIG. 7B, the optical signal willexperience a phase shift, the amount (in degrees, for example) willdepend, at least in part, on the position of the phase shift element 53and the width of the element 53 encountered by the optical signal.

If desired, phase shifter 51 could be arranged so that actuator 55 isdisposed between arms 15, 17.

Generally, phase shift element 53 is constructed of opticallytransparent material such as, for example, silicon. The element 53preferably has certain optical qualities, in particular, a refractiveindex different from that of the waveguides 117, 217 and from the mediumprovided in the trench 59, which may be air or a vacuum, for example.Light passing between the waveguides 117, 217 and through the phaseshift element 53 will experience a change in velocity and thus a phaseshift due to the difference in refractive indices. Since the phase oflight passing through phase shift element 53 is affected by both thephase shift element's index of refraction and its width, these valuescan be selected to impart the desired phase shift(s). It is presentlypreferable for the phase shift element 53 to introduce a phase shiftranging from approximately 0° when the element 53 is not in the opticalpath, to approximately 180° (π) when the element 53 is in the opticalpath 53. For a tapered or wedge-shaped element 53, such as depicted inFIG. 2, a range of phase shifts may be selected, depending upon the sizeand shape of the element 53. Alternative embodiments of the phase shiftelement 53 in accordance with the present invention may also provide arange of phase shift values (see, e.g., FIGS. 9A-9C).

With continued reference to FIGS. 2 and 6A, phase shift element 53 ispreferably tapered, or wedge-shaped with a height h sufficient tocompletely intercept and thereby shift light passing between waveguides117 and 217 when the phase shift element 53 is positioned in the opticalpath.

The phase shift element 53 has a length l that is preferably minimizedto reduce the distance by which the phase shift element 53 is movedbetween the first and second positions (i.e., positions in and out ofthe optical path, for example), or the distance by which the element 53is moved to change the phase shift from one value to another. Theminimized length l may also reduce the power needed to cause the phaseshift element 53 to move into and out of the optical path and improvesthe switch's response speed.

Tapered phase shift element 53 preferably has a maximum width t. Sincethe width t of the phase shift element 53 directly affects the insertionloss through the MZI 13 and switch 7, a thinner phase shift element 53may be preferred. Optical loss of light due to light diffraction in thetrench 59, also can be minimized by having the smallest possible phaseshift element width t.

With continued reference to FIG. 2, a preferred construction of an MZI13 and switch 7 in accordance with the present invention will now bediscussed in detail. The switch 7 and MZI 13 are generally constructedas waveguides, such as depicted in FIG. 4, which depicts anillustrative, non-limiting cross-sectional representation of a stronglyconfined waveguide 35. The waveguide 35 is constructed on a substrate 37and is comprised of a relatively high (e.g., n=3.5) refractive indexcore 39 surrounded on at least two sides (in the horizontal direction inFIG. 4) by a relatively low refractive index medium 41 such as air. Thecore 39 is sandwiched between upper and lower cladding layers 43, 45.

The present invention contemplates waveguides constructed in lithiumniobate, silica/glass, and other semiconductor materials provided thatstrong confinement (at least in the horizontal direction in FIG. 4) isachieved.

With continued reference to FIG. 4, the waveguide 35 there depicted incross-section may comprise either a photonic-well or a photonic-wirewaveguide. Exemplary photonic-wire and photonic-well devices arerespectively disclosed in U.S. Pat. Nos. 5,878,070 and 5,790,583, theentire disclosure of those patents being incorporated by referenceherein. The waveguide 35 can be formed of semiconductor materials foron-chip integration with other devices such as a semiconductor laser. Awafer epitaxial growth process, or other now known or hereafterdeveloped semiconductor fabrication process, may be used to form thevarious semiconductor layers of the waveguide 35 on the substrate 37. Asdepicted in FIG. 4, a lower cladding layer 45, preferably of SiO₂, isformed on the substrate 37, preferably silicon (e.g., Si) or quartz. Acore 39 is formed on the first cladding layer 45 and, by way ofnon-limiting example, can be made from SiO₂. An upper cladding layer 43, also preferably of SiO₂, is formed on the core 39.

For a photonic-wire construction, the refractive index of the core 39 isgenerally greater than that of all of the upper and lower claddinglayers 43, 45, and the surrounding medium 41. In a photonic-wirewaveguide 35, the upper and lower cladding layers 43, 45 have a very lowrefractive index as compared to the refractive index of the core 39 andthus strongly confine photons in all directions about the waveguide core39. Typical low refractive index mediums for use in practicing thepresent invention have refractive index below about 2.0, preferablybelow 1.6, such as from about 1.5 to about 1.0. The ratio of therefractive indices between the core 39 and each of the upper and lowercladding layers 43, 45 and the surrounding medium 41 is preferablylarger than about 1.3.

For a photonic-well construction, the refractive index of the core 39 isgenerally greater than that of the surrounding medium 41, with the upperand lower cladding layers 43, 45 having a refractive index close to thatof the core 39 and thus weakly confine photons within the waveguide 35in the vertical direction. However, strong lateral confinement is stillprovided by the difference between refractive index of the core 39 andthe relatively low refractive index cladding medium 43, 45 laterallysurrounding the core 39. In a photonic-well waveguide 35, the claddinglayers 43, 45 may have a refractive index of about 3.17 as compared tothe refractive index of 1 for air or of 1.5 for silica. The refractiveindex of cladding layers 43, 45 is slightly less than the refractiveindex of core 39, which is preferably about 3.4.

Presently it is believed that silica-based (SiO₂) materials are thoughtto be preferable for constructing waveguides for the various embodimentsof the present invention. In particular, core 39 might include germaniumoxide doped silica deposited atop a silica substrate 37, while cladding43 and 45 may include boron-phosphine doped silica glass. Othermaterials which could be used for the core 39 include indium phosphideand gallium arsenide, and the cladding 43, 45 could be made with indiumphosphide, gallium arsenide, aluminum oxide, silicon nitride orpolymers, or some combination thereof.

The core 39 can be rectangular, with sides running from approximately3-10 μm thick and approximately 3-15 μm wide. More preferably, the core39 is square, with sides from approximately 6-8 μm thick andapproximately 6-14 μm wide. The upper and lower cladding layers 43, 45adjacent to core 39 can be approximately 3-18 μm thick, and arepreferably approximately 15 μm thick.

The present invention will work with both weakly-confined waveguides andstrongly-confined waveguides. Presently, use with weakly-confinedwaveguides is preferred.

Referring next to FIG. 3, a 2×2 optical switch 7 having two branches 9,11 and constructed in accordance with another embodiment of the presentinvention is depicted. The switch 7 includes a MZI 13 having first andsecond arms 15, 17 optically connecting an input coupler 19 and anoutput coupler 21 along an optical path direction of the switch 7,generally indicated by arrows A and B. The couplers 19, 21 depicted inFIG. 3 may be co-directional, 3 dB couplers, by way of non-limitingexample. Alternatively, Y-branches or multi-mode interferometer (MMI)couplers may be provided, as a routine matter of design choice.

The switch 7 may receive an optical signal input from either one of twooptical sources 23, 25, each of which may, by way of example only, andnot in a limiting sense, include a laser, fiber-optic cable, or otherupstream (along the optical path direction) light generating or lightpropagating device or system. A first optical signal may be directedinto an input 27 of the switch 7 by first optical source 23. The firstoptical signal may comprise a single- or multi-wavelength signal, and asexplained hereafter may be selectively switched in a known manner toeither output 31 or 33. Similarly, and alternatively, a second opticalsignal may be directed into an input 29 by a second optical source 25,and may also be selectively switched to either of output 31 or 33. Theoptical signal output from the switch 7 via outputs 31 and 33 are sineand cosine functions of wavelength, respectively (as described ingreater detail below), and thus are complementary.

With continued reference to FIG. 3, an optical signal from an opticalsource 25 or 27 may pass through an input coupler 19 which functions asa 50:50 splitter to direct approximately one-half (in terms of signalamplitude or power) of the input optical signal to each of the first andsecond arms 15, 17 of the MZI 13. The split optical signal passesthrough each of the first and second arms 15, 17, is recombined by anoutput coupler 21, and is then output from either output 31 or 33,according to the phase shift introduced in the optical signal by the MZI13. As described above, actuating the phase shifter 51 causes an opticalsignal propagating in arm 17 to undergo a phase shift. The non-phaseshifted optical signal (propagating through first branch 9 and first arm15, for example) combines with the phase shifted optical signal(propagating through second branch 11 and second arm 17, for example)via the output coupler 21. In known manner the optical signal may beswitched between the two output ports 31 and 33 of the switch 7according to the relative phase of the optical signal propagating in andthrough the two arms 15, 17 of the MZI 13.

The two output ports 31, 33 of the switch 7 are complementary andrespectively provide in known manner an optical signal of the formP_(A)=sin²(Δφ/2) and P_(B)=cos²(Δφ/2). Consequently, the relative phaseshift between the two arms 15, 17 of the interferometer 13 willdetermine how the optical signal is switched between the two outputports 31, 33 of the switch 7.

A tapered phase shift element 53 may require a relatively preciseactuator 55 to effect the desired movement of the element 53 into andout of the optical path, or within the optical path, as describedherein. For a tapered phase shift element 53, the actuator 55 must causethe phase shift element 53 to move from position out of the optical pathto a particular and relatively precise position so that the opticalsignal passes through the phase shift element 53 at a particularthickness and the desired phase shift is introduced into the opticalsignal. For example, consider a tapered phase shift element 53 having aπ/(50 μm) (maximum phase shift amount over length) phase shift element53 positioned so as to introduce a π/6 phase shift into an opticalsignal. If it is desired to change that phase shift from π/6 to π/3, itwill be necessary to increase the phase shift by π/6. This will requiremoving the phase shift element 53 by approximately 8 μm, as show clearlyin equation (2); which may be used to calculated the amount of movementrequired of the element 53 for a desired phase shift. $\begin{matrix}{\frac{\left( {\pi/3} \right) - \left( {\pi/6} \right)}{\pi/\left( {50\quad µ\quad m} \right)} = {8\quad µ\quad m}} & (2)\end{matrix}$

It will be appreciated that such small movement requires precise controlof the position of the wedge-shaped phase shift element 53.

One alternative to a more accurate actuator 55 is a more graduallysloping phase shift element 53. For example, halving the phase shiftelement's taper will double the distance by which the phase shiftelement 53 would have to be moved to cause the same magnitude phaseshift. This effectively increases the accuracy of the actuator 55. Forexample, a phase shift element 53 having a length approximately equal to10 μm and constructed to introduce a πphase shift into an optical signalwould have sides 87, 89 that slope at a rate approximately twice that ofa 20 μm phase shift element 53.

Other than inducing a phase shift, the material used in the phase shiftelement 53 should not significantly alter (i.e., absorb) thecharacteristics of the light which passes therethrough.

Referring next to FIG. 8, the effect of a tapered phase shift element 53on an optical signal or light beam 77 is there depicted. Since theamount of phase shift introduced into an optical signal by the phaseshift element 53 is determined, at least in part, by the thickness ofthe element 53, an element 53 having a variable thickness may be used tointroduce a selectable, variable phase into an optical signal.

The tapered sides 87, 89 of the phase shift element 53 may cause anoptical signal to experience a non-uniform phase shift over the width ofthe optical signal light beam 77.

Since the amount of phase shift introduced into the optical signaldepends, at least in part, upon the thickness of the phase shift element53, the light beam will encounter varying thicknesses simply because thelight beam has a finite width. Consequently, an edge 177 of the lightbeam encountering a wider part of the phase shift element 53 willexperience a greater phase shift than an edge 277 of the light beamencountering a narrower part. If the width of the light beam 77 isrelatively small in comparison to the length of the phase shift element53, the difference in phase experienced at the edges 177, 277 of thelight beam 77 may be too small to adversely effect further transmissionof the optical signal and thus may not require correction orcompensation.

If, however, correction or compensation is desired, one way to reducethe difference in phase shift would be to use a very gradually taperedphase shift element 53 so that the light beam 77 experiences relativelynegligible difference in thickness of the element 53 over the width ifthe light beam 77 thus providing a more homogeneously phase shiftedoptical signal. Such a phase shift element 53 could be capable ofproducing as wide a range of phase shifts as a more sharply taperedphase shift element 53, although more movement of the phase shiftelement 53 would be required.

With reference to FIGS. 2, 5A, 5B and 6, phase shift element 53 ispreferably oriented approximately perpendicular to the optical pathdirection, indicated by arrow A in FIG. 2. Phase shifter 51 enables auser to select, within limits, the amount by which the phase of lighttraveling along the second arm 17 is changed.

The tapered phase shift element 53 can have a width ranging fromapproximately submicron-size at the tip to 100 μm at the widest portion,a length ranging from approximately 10-100 μm, and a height ofapproximately 1-8 μm, and can be made from any sufficiently rigid andlight material. Preferably, the tapered phase shift element 53 istriangular, has a tip width of approximately submicron size, a maximumwidth of 30-40 μm,, and a length of approximately 30-40 μm, and is madefrom silicon. By way of non-limiting example, other materials such aspolymers, metallic materials or dielectric films also could be used.

An alternative embodiment of a phase shift element 53 in accordance withthe present invention is depicted in FIGS. 9A-9C. The stepped phaseshift element 153 consists of two or more different rectangular phaseshift regions 93, 93′, 93″ having different thicknesses, t, t, t″. Sincethe phase shift of light passing through each phase shift region 93,93′, 93″ is a function of the phase shift region's thickness, it will beunderstood that thicker phase shift regions introduce a greater phaseshift than thinner phase shift regions. Instead of allowing an infiniterange of phase shifts from 0-180°, as is possible with a tapered phaseshift element 53, this arrangement provides for a discrete number ofphase shifts.

The number of phase shifts possible using a stepped phase shift element153 as depicted in FIGS. 9A-9C will correspond to the number of phaseshift regions 93, 93′, 93″. For example, a six-step phase shift elementcould provide phase shifts approximately equal to π/6, π/3, π/2, 2π/3,5π/6 and π. When configured as depicted in FIG. 9A, or alternatively,with the smallest thickness being located near the link 57, the steppedphase shift element 153 provides monotonic phase shifting of an opticalsignal. Alternatively, non-monotonic phase shifting may also beprovided, as a routine matter of design choice.

When viewed from one end, such as depicted in FIG. 9C, for example, thestepped phase shift element 153 can be seen to have a number of phaseshift regions 93, 93′, 93″ all arranged symmetrically about a commoncenter plane 95 defined through the element 153. Alternatively, thestepped phase shift element 253 may have a stepped side 162 and a flatside 160, as depicted in FIG. 9B, with either side serving as an inputor output for the optical signal as it propagates between the waveguides117, 217 of the interferometer arm 17.

Individual phase shift regions 93, 93′, 93″ of the stepped phase shiftelement 153, 253 need not be arranged either symmetrically. For example,phase shift regions 93, 93′, 93″ could be arranged so that the mostfrequently used phase shift regions are adjacent to one another (notshown). This arrangement will reduce the distance by which the phaseshift element 153, 253 would have to be moved to place those most usedphase shift regions in the optical path. Since the phase shift element153, 253 has to be moved a shorter distance, the phase shifter'sresponse time would be improved.

The stepped phase shift element 153, 253 can be fabricated either as asingle integral piece or an assembly of several suitably-aligned piecesadhered or bonded together. Fabricating a single integral piece may bepreferable because that avoids the need to align precisely the assembledpieces, and also avoids deformations in the optical material which mightbe caused by the adhering or bonding of the several pieces.

Another benefit to using a stepped phase shift element 153, 253 is thata less precise actuator 55 may be needed, since the minimum distance bywhich the phase shift element 153, 253 will have to be shifted isapproximately equal to the distance between the centers of two adjacentphase shift regions. Given that the phase shift regions are themselvessomewhat wider than the width of the light beam 152, the minimum amountby which the actuator 55 would move the phase shift element 153, 253could be somewhat larger than the beam of light.

To ensure that the light beam 152 does not simultaneously encounter twodifferent, adjacent phase shift regions, the length of each region ispreferably no less than the width of the waveguide 117, 217.

Actuator 55 serves to move the phase shift element 53, 153, 253 into andout of the optical path in the region or trench 59. While any suitabletype of actuator could be used to implement this invention, it ispresently thought that either an electrothermal or electromechanicaltype actuator would be preferred. Both types of actuators are forgeneral purposes known in the art, and so will not be described inprecise detail. For the purposes of this invention, it will beappreciated that any actuator could be used which sufficiently changesits size in response to the application of energy. In some cases, aswill be evident from the following discussion, large displacements ofthe phase shift element 53, 153, 253 may be necessary. There,electrothermal actuation may be preferred.

The width of the trench 59 between the waveguides 117, 217 is preferablyminimized to reduce diffraction of the optical signal as it propagatesacross the trench 59 and between the waveguides 117, 217. For example,and as depicted in FIGS. 14A and 14B, greater diffraction of the opticalsignal is likely to occur with greater trench widths. Since the lightdiffracts more as the trench width increases, optical signal loss willoccur from waveguide 117 to waveguide 217. It is therefore preferable toprovide a trench 59 having as short a width as practical and to positionthe ends 63, 163 (see, e.g., FIGS. 7A and 7B) of waveguides 117, 217 asclose to each other as possible.

The trench 59 can be from approximately 8-40 μm wide. Preferably, thetrench is approximately 12-20 μm wide.

There are several ways to control diffraction of the light as it crossesthe trench 59. Diffraction can be controlled by separating the ends 63,163 of waveguides 117, 217 by a distance only slightly greater than thewidest part of the phase shift element 53, as depicted in FIGS. 7A and7B. It is thus desirable to provide as narrow a trench 59 as possible tominimize light diffraction losses as light propagates through and acrossthe trench 59. Trench widths ranging from approximately 10 to 35 μm arepresently thought to be preferable.

At the same time there are factors which limit how narrow a trench 59can be provided. A narrow trench 59 may complicate aligning the facingwaveguides 117, 217, and may not be able to accommodate a phase shiftelement 53 of width sufficient to apply the maximum desired phase shiftfor the tuning range of interest.

As depicted in FIG. 10A, diffraction losses in wider trenches can bereduced by increasing the waveguide widths using tapers 90, 190integrally formed as part of the waveguides 117, 217. Alternatively, thetapers 90, 1,90 may be separate components attached to the waveguides117, 217.

Moreover, only one of the two tapers 90, 190 could be provided. In suchan embodiment, only output waveguide 217 would be provided with taper190, so that light would leave input waveguide 117, pass through trench59, enter taper 190 and from there pass into waveguide 217.

With reference to FIG. 10B, a lens 99 may be provided at an output ofwaveguide 117 to minimize the diffraction of light as it exits thewaveguide 117 and propagates through and across the trench 59. Whilesuch a lens 99 could be formed in a variety of ways, an etched lens ispresently preferred.

It also may desirable for the trench 59 to be in lined relative to theaxis along which the waveguides 117, 217 are arranged (not shown).Preferably the trench 59 is inclined relative to that axis at an angleranging from approximately 4° to 8°, and more preferably, fromapproximately 5° and 7°, and most preferably, approximately 6°. Thisgeometry prevents light reflecting off the phase shift element 53 frombeing directed back into waveguide 117.

A tapered phase shift element 53 may cause the optical signal to beoffset from its initial optical path, i.e., defined as the opticalsignal exits waveguide 117, due to prismatic effects of the taperedphase shift element 53. As depicted in FIG. 11, waveguide 217 may bepositioned with respect to waveguide 117 to accommodate that offset inthe optical path.

Turning again to FIG. 2, actuator 55 can be driven to selectively varythe position of variable phase shift element 53 in trench 59. Dependingupon the position of the phase shift element 53, a phase shift rangingfrom approximately 0° to approximately 180°(π) may be introduced into anoptical signal propagating in and through the MZI 13. The phase shiftedoptical signal propagating through second arm 17 thereafter combineswith the non-phase shifted optical signal propagating through first arm15 at waveguide 49′. Depending on the relative phase shift in theoptical signal propagating in and through each arm 15, 17, the opticalsignal output from the MZI 13 via output 31 will vary in amplitude fromapproximately 0 (for a 180° phase shift) to approximately 100% (for a 0°phase shift) of the amplitude of the signal entering the device at input27. For example, when a 0° phase shift is applied, i.e., when the phaseshift element 53 is not positioned in the optical path, the two opticalsignals combined in waveguide 49′ will constructively interfere witheach other to provide an output signal approximately equal (in phase andamplitude, for example), to the input signal. On the other hand, if a180° phase shift is applied, the two optical signals combined inwaveguide 49′ will destructively interfere with each other to provide nooutput signal. It will be obvious to persons skilled in the art and fromthe disclosure provided herein that phase shifts between 0° and 180°will similarly effect the phase and magnitude of the output of the MZI13 thereby providing an analog output from the MZI 13.

As shown in FIGS. 2, 3, 5A and 5B, the phase shift element 53 is affixedto actuator 55 by link 57 and is arranged to move reciprocally withoutinterference in and along trench 59. In an embodiment of the presentinvention, and as depicted in FIG. 2, the phase shift element 53 may beselectively moved into and out of the optical path along a linegenerally parallel with a surface 159 defined in the trench 591 Link 57is preferably made from a light-weight, stiff material. The actuator 55thus enables selective movement of the phase shift element 53 into andout of the optical path defined through the trench 59 by the waveguides117, 217, and selective positioning of the phase shift element 53 to anyof a plurality of positions within the optical path. A phase shiftranging from 0° to 180° may thus be introduced in an optical signalpassing through the phase shift element 53 depending upon the positionof the element 53.

It should be understood that the direction of movement of the phaseshift element 53 is not limited to movement in and along the trench 59.Phase shift element 53 may be moved in any direction which guides itinto and out of the optical path, or which provided selectivepositioning within the optical path. With reference to FIG. 6, phaseshift element 53 is connected to actuator 155 by link 157. Phase shiftelement 153 can be reciprocated by the actuator 155 into and out of theoptical path, as indicated by arrow D, along a line generallyintersecting the surface 159, or similarly, along a diagonal lineintersecting the surface 159.

Actuator 55 serves to move the phase shift element 53 into and out ofthe optical path. While any suitable actuator could be used to implementthis invention, it is presently thought that either an electrothermal orelectromechanical type actuator would be preferred.

Electrothermal actuators are in general known in the art, and thereforewill not be described in precise detail. For the purposes of thisinvention, it will be appreciated that any electrothermal actuator couldbe used which sufficiently changes its size in response to theapplication of energy.

One benefit to using electrothermal actuators is that such actuators maybe latching-type devices maintain its position without the continuousapplication of energy.

Although electrothermal actuators are relatively simple to manufactureand operate, they are relatively slow to act, and introduce heat to thesystem. Thus, other actuators may be used instead.

Electrostatic actuators could be used to move the phase shift element.Benefits of electrostatic actuators include high operating speed, lowenergy consumption, and minimal system heating.

Another aspect of the present invention compensates for optical returnloss (ORL) caused when an optical signal passes between materials havingdifferent refractive indices, which may occur here where the opticalsignal passes from waveguide 117, across trench 59, through phase shiftelement 53, and into waveguide 217. The difference in refractive indicesof those materials, particularly between the core 39 of waveguides 117,217, the medium 61 provided in trench 59, and the phase shift element53, may cause part of the optical signal (in terms of optical power) tobe reflected by the phase shift element 53 and propagate back into thewaveguide 117 and along the optical path, for example. That reflectedsignal can disadvantageously reflect back to and possible destabilizethe optical signal source.

With reference now to FIG. 12, by suitably angling the ends 63, 163which border trench 59, any reflected signal is directed away from thewaveguide core 39 and toward the cladding 43 or 45 (see, e.g., FIG. 4),thereby preventing the reflected light from interfering with the opticalsignal being guided by and propagating in the waveguides 117, 217′. Inan embodiment of the present invention, the ends 63, 163 could bedisposed at an angle ranging from about 6° to 10°, and more preferably,about 8°, to minimize the loss due to a reflected optical signal. It ispreferable to keep the ends 63, 163 substantially vertical relative tothe substrate 37, and to bevel the ends 63, 163 along a planeperpendicular to the plane of the substrate 37. A further benefit tothis arrangement is the destruction of the Fabry-Perot cavity whichwould be formed were the ends 63, 163 perpendicular to one another.

In another aspect of the present invention, optical return loss may befurther minimized by applying an antireflective coating (not shown) toat least one of the waveguide ends 63, 163.

The MZI 13 of the present invention, and an optical switch 7 formedtherefrom, may be monolithically formed or assembled using a flip-chipmanufacturing technique, the latter being generally depicted in FIGS.13A and 13B. In flip-chip manufacturing, the waveguides 49, 49′ andtrench 59 are monolithically formed on a first chip 200 using knownsemiconductor fabrication techniques and processes (e.g., deposition,etching, etc.). The phase shift element 53, actuator 55 and spacers 75are formed on a second chip 210. Prior to assembly, the two chips 200,210 are oriented to face each other, and aligned so that correspondingparts (e.g., phase shift element 53 and trench 59) of the chips opposeone another. Spacers 75 regulate the distance between chips 200, 210 asthey are joined, and keep the chips from being pressed too closetogether. The spacers 75 may also be used to insure that the chips 200,210 are joined in proper registration with each other. The chips arethen joined in known fashion.

Alternatively, in another embodiment of the present invention, the MZI13 of the present invention, and switches 7 constructed therefrom, maybe constructed by monolithically forming the various parts (e.g.,waveguides, phase shifter (phase shift element, link and actuator)). Insuch an embodiment, the various parts of the MZI 13 and switch 7 areformed on a single substrate 37 through the selective deposition andremoval of different layers of material using now known or hereafterdeveloped semiconductor etching techniques and processes. One of thebenefits of monolithic fabrication is that it avoids the need toregister the different components before the two substrates are joined.

Referring to FIGS. 15A and 15B, both a flip-chip and monolithicallyformed optical switches 7 in accordance with the present invention arethere respectively depicted. Both figures depict connection of theoptical switch 7 to external optical components such as, for example,optical fibers 67, such that waveguide cores 39 optically align withfiber cores 65. Each optical fiber 67 is supported by a grooved member69, and secured in place using a fiber lid 71. A glass cover 73 protectsthe underlying components. Alternative ways of securing the opticalfibers, or of using other light pathways, also could be used.

One difference between the two fabrication techniques is the location ofthe phase shifter 51 above the waveguides for flip-chip and within thesubstrate 39 for monolithic.

The above-described semiconductor materials and relative refractiveindices are illustrative, non-limiting examples of embodiments of thewaveguide structure of the present invention.

Thus, while there have been shown and described and pointed out novelfeatures of the present invention as applied to preferred embodimentsthereof, it will be understood that various omissions and substitutionsand changes in the form and details of the disclosed invention may bemade by those skilled in the art without departing from the spirit ofthe invention. It is the intention, therefore, to be limited only asindicated by the scope of the claims appended hereto.

It is also to be understood that the following claims are intended tocover all of the generic and specific features of the invention hereindescribed and all statements of the scope of the invention which, as amatter of language, might be said to fall therebetween.

What is claimed is:
 1. A Mach-Zehnder interferometer comprising: acontinuous arm defining a first optical path; a non-continuous armhaving a first part and a second part with a trench definedtherebetween, said non-continuous arm defining a second optical path;and a phase shifter selectively positioned within the second opticalpath; said phase shifter being selectively movable between a firstposition, in which the phase shifter introduces no phase shift to anoptical signal traveling along the second optical path, and at least asecond position at which the phase shifter introduces a phase shift tothe optical signal traveling along the second optical path, relative toan optical signal propagating through the first optical path.
 2. AMach-Zehnder interferometer according to claim 1, wherein said phaseshifter is dimensioned to introduce a phase shift ranging fromapproximately 0° to approximately 180° into the optical signal when saidphase shifter is in said second optical path.
 3. A Mach-Zehnderinterferometer according to claim 1, wherein said optical phase shiftercomprises: a phase shift element; and an actuator coupled to the phaseshift element for causing selective movement of the phase shift elementbetween the first position in which said phase shift element is out ofsaid second optical path, and the second position in which said phaseshift element is in said second optical path so as to introduce thephase shift in the optical signal propagating along said second opticalpath.
 4. A Mach-Zehnder interferometer according to claim 3, whereinsaid phase shift element is sized and shaped to introduce a phase shiftranging from approximately 0° to approximately 180° into the opticalsignal when said phase shift element is in said second optical path. 5.A Mach-Zehnder interferometer according to claim 4, wherein said phaseshift element is a wedge-shaped prism.
 6. A Mach-Zehnder interferometeraccording to claim 4, wherein said phase shift element has a steppedprofile.
 7. A Mach-Zehnder interferometer according to claim 6, whereinsaid phase shift element has a single-sided stepped profile.
 8. AMach-Zehnder interferometer according to claim 1, wherein said firstpart and said second part of said non-continuous arm are separated bysaid trench and a distance of not more than approximately 8-40 μm.
 9. AMach-Zehnder interferometer according to claim 1, wherein said firstpart and said second part of said non-continuous arm are separated bysaid trench and a distance of not more than approximately 1-20 μm.
 10. AMach-Zehnder interferometer according to claim 1, wherein said trenchhas a substantially constant depth.
 11. A Mach-Zehnder interferometeraccording to claim 1, wherein said trench has a variable depth.
 12. AMach-Zehnder interferometer according to claim 1, wherein a surface isdefined in said trench and wherein said phase shifter is selectivelymovable into and out of said second optical path along a line generallyparallel with said surface.
 13. A Mach-Zehnder interferometer accordingto claim 1, wherein a surface is defined in said trench and wherein saidphase shifter is selectively movable into and out of said second opticalpath along a line generally intersecting said surface.
 14. AMach-Zehnder interferometer according to claim 1, wherein saidnon-continuous arm has an index of refraction and wherein said phaseshifter has an index of refraction different from said non-continuousarm index of refraction.
 15. A M×M optical switch for receiving anoptical signal from an optical source and for switching the opticalsignal to one of M outputs of said switch, said switch comprising: aMach-Zehnder interferometer comprising: a continuous arm defining afirst optical path; a non-continuous arm having a first part and asecond part with a trench defined therebetween, said non-continuous armdefining a second optical path; and a phase shifter positioned withinthe second optical path, said phase shifter being selectively movablebetween a first position, in which the phase shifter introduces no phaseshift to an optical signal traveling along the second optical path, andat least a second position at which the phase shifter introduces a phaseshift to the optical signal traveling along the second optical path,relative to an optical signal propagating through the first opticalpath.
 16. An optical switch according to claim 15, wherein said phaseshifter is dimensioned to introduce a phase shift ranging fromapproximately 0° to approximately 180° into the optical signal when saidphase shifter is in said second optical path.
 17. An optical switchaccording to claim 15, wherein said optical phase shifter comprises: aphase shift element; and an actuator coupled to the phase shift elementfor causing selective movement of the phase shift element between thefirst position in which said phase shift element is out of said secondoptical path, and the second position in which said phase shift elementis in said second optical path so as to introduce the phase shift in theoptical signal propagating along said second optical path.
 18. Anoptical switch according to claim 17, wherein said phase shift elementis sized and shaped to introduce a phase shift ranging fromapproximately 0° to approximately 180° into the optical signal when saidphase shift element is in said second optical path.
 19. An opticalswitch according to claim 18, wherein said phase shift element is awedge-shaped prism.
 20. An optical switch according to claim 18, whereinsaid phase shift element has a stepped profile.
 21. An optical switchaccording to claim 18, wherein said phase shift element has asingle-sided stepped profile.
 22. An optical switch according to claim15, wherein said first part and said second part of said non-continuousarm are separated by said trench and a distance of not more thanapproximately 8-40 μm.
 23. An optical switch according to claim 22,wherein said first part and said second part of said non-continuous armare separated by said trench and a distance of not more thanapproximately 12-20 μm.
 24. An optical switch according to claim 15,wherein said trench has a substantially constant depth.
 25. An opticalswitch according to claim 15, wherein said trench has a variable depth.26. An optical switch according to claim 15, wherein a surface isdefined in said trench and wherein said phase shifter is selectivelymovable into and out of said second optical path along a line generallyparallel with said surface.
 27. An optical switch according to claim 15,wherein a surface is defined in said trench and wherein said phaseshifter is selectively movable into and out of said second optical pathalong a line generally intersecting said surface.
 28. An optical switchaccording to claim 15, wherein said non-continuous arm has an index ofrefraction and wherein said phase shifter has an index of refractiondifferent from said non-continuous arm index of refraction.